Anisotropically conductive materials are typically comprised of an insulative medium having a plurality of spaced-apart, electrically conductive paths extending therethrough along a first direction so that materials exhibit a high conductivity along the first direction, and a high electrical resistance along all other directions. Typically, the electrically conductive paths in most anisotropically conductive materials extend parallel to the z or thickness direction of the material. Thus, such anisotropically conductive materials exhibit a preferential conductivity along their thickness and a high electrical resistance laterally through the x-y plane of the material.
A common use of anisotropically conductive materials is to electrically connect a pair of devices, each having an array of opposed, spaced-apart contacts lying in a separate one of a pair of parallel, spaced-apart planes. To connect the two devices, the anisotropically conductive material is sandwiched therebetween so a separate one of the conductive paths through the material extends between a separate one of the contacts on each device. Since the conductive paths in the anisotropically conductive material are laterally separated from each other by an insulative medium, a connection between one and only one contact on the pair of devices is provided by the anisotropically conductive material. Undesirable cross-connections between two or more contacts on the same device are thus avoided.
An early proposal for fabricating a sheet of anisotropically conductive material, having a preferential conductivity along its thickness direction, is disclosed in U.S. Pat. No. 4,209,481, issued June 24, 1980. In accordance with the teachings of that patent, a quantity of electrically conductive, ferromagnetic wires, each of a length at least as great as the desired thickness of the sheet, is mixed in a liquid insulative polymer, such as silicone rubber. The polymer is spread into a sheet which is then cured (permitted to vulcanize or dolidify) in the presence of a magnetic field whose lines of force are parallel to the thickness direction of the sheet. When subjected to the magnetic field, the wires experience a magnetic force which causes them to rotate and align themselves in bundles parallel with the lines of the field. The bundles of wires extend between the opposed major surfaces of the polymer sheet so as to provide a conductive path therebetween.
As taught in the aforementioned patent, the magnetic field applied to the polymer varies spatially, that is, the field is comprised of alternating regions of high and low field strength which are spatially periodic. In other words, each region of high and low field strength is separated from a region of like field strength by a uniform distance or pitch. It is believed that the reason for employing a spatially varying field is to avoid the problem of the wires becoming entangled with each other as they align with the lines of the field. As the wires in the polymer align themselves with the field, more energy is stored in the regions of high strength than in the regions of low field strength. In this type of physical system, the wires in the magnetic field always seek to increase the energy of the system. Hence, the wires in the polymer will gather in the regions of high field strength. By making the spacing between adjacent regions of the same strength greater than the length of the wires, the wires will separate a sufficient distance to avoid entangling with each other as they align with the lines of the magnetic field.
A disadvantage associated with fabricating a sheet of anisotropically conductive material in this fashion is that the spacing between adjacent bundles of wires is always greater than the length of the wires, and hence the thickness of the sheet. The ability of a sheet of anisotropically conductive material produced in the above manner to provide a one-to-one electrical connection between the contacts on two devices becomes limited as the spacig between the contacts on each device becomes smaller. Currently, the trend in the electronics industry is towards producing smaller devices which have more closely spaced contacts. Thus it is necessary for the lateral spacing between the conductive paths in the anisotropically conductive polymer to be very small if the material is interconnect devices having very closely spaced leads.
A more recent proposal for fabricating a sheet of anisotropically conductive material is to mix a plurality of electrically conductive, ferromagnetic spheres, each typically of a diameter much less than that of the desired sheet thickness, in an insulative polymer. The sphere-filled polymer is spread in a sheet which is cured in a magnetic field whose lines are parallel to the thickness direction of the sheet. When subjected to the magnetic field, the spheres align themselves contiguous with each other in chains because in such a configuration, the energy stored in the field reaches a local maximum value.
The advantage obtained in using spheres is that they do not tangle with each other as they align in chains parallel to the lines of the field. As a result, there is no need to employ a spatially varying field having alternating, uniformly spaced regions of high and low field strength in order to cure a sphere-filled polymer, in contrast to the wire-filled polymer. Rather, a spatially uniform field is employed to cure the sphere-filled polymer because the spheres tend to laterally separate to a lesser extent in such a field as they would in a spatially varying field. The smaller the extent of the lateral separation between the spheres, the smaller the lateral spacing between the chains of spheres. Thus, curing the sphere-filled polymer in a spatially uniform field may result in a smaller lateral spacing between the chains of spheres which is desirable.
One of the difficulties incurred with curing the sphere-filled polymer in a spatially uniform magnetic field is that the lateral spacing between the chains of spheres cannot be well controlled. Typically, the chains of spheres are randomly located. When using the sphere-filled, anisotropically conductive material to interconnect two devices, each having closely spaced contacts, the random spacing between the chains of spheres may result in one of the contacts on a device not being in contact with a chain of spheres. As a result, one or more of the contacts on each device may not be connected to a corresponding contact on the other device through a conductive path in the sheet of anisotropically conductive material.